ONO stands for Or Nearest Offer. Like OVNO, it's shorthand in classified ads meaning the seller will consider offers below the asking price.
The practical difference from OVNO is mostly in tone: ONO is generally read as leaving more room for negotiation, since it doesn't specify the offer has to be "very near" the asking price, just the nearest one the seller gets. In practice, how much room there really is comes down to the individual seller rather than the abbreviation itself.